1. Field of the Invention
The present invention relates to dose control in a lithographic projection apparatus.
2. Description of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics and catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Any refractive, reflective or catadioptric elements in the radiation or illumination systems may be based on a substrate of glass or another suitable material, and may be provided with either single- or multi-layer coatings as desired. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M less than 1, the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
A step-and-scan exposure apparatus is described in WO 97/33204. In this apparatus, reticle masking blades are used to define a slit through which an illumination beam passes to illuminate a rectangular portion of a reticle, an image of which is projected onto a wafer with a magnification M. The reticle is moved so that the illuminated portion scans the whole pattern to be imaged. The wafer is moved synchronously at a speed M times that of the reticle in the opposite direction to the reticle so that the whole pattern is imaged onto the wafer. At the end of a scan the wafer is stepped to the beginning of the next field to be illuminated, and the illumination process is repeated.
It is important to ensure that the entire field is properly and uniformly illuminated. This requires that the slit, defined by the reticle masking blades, is constantly and uniformly illuminated throughout the scan. Control of the exposure in a scanning exposure system has therefore been a problem.
It is an object of the present invention to provide a lithography apparatus having improved control over the field illumination uniformity.
The present invention relates to dose control in a lithographic projection apparatus having:
a radiation system for supplying a projection beam of radiation;
a first object table movable in at least a first direction, the scan direction, and provided with a mask holder for holding a mask having a mask pattern;
a second object table movable in the first direction and provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate with a magnification M.
According to the present invention, this and other objects are achieved in a lithographic projection apparatus as defined above, in which:
the radiation system is adapted to supply a projection beam whose cross-section in the plane of the mask is smaller than the mask pattern, the radiation system including movable beam masking means for controlling the cross-section of the projection beam;
control means for causing the beam masking means to be closed prior to, and open at, the beginning of a scan; and
compensation means for compensating for a variation in radiation intensity of the projection beam on the mask whilst the beam masking means are opening.
The present inventors have determined that exposure (dose) errors occur at the beginning and end of the scan and are caused by reflections from the reticle masking blades. The present invention therefore provides means for compensating for the increased illumination as the slit defined by the beam masking means (e.g. reticle masking blades) is opened and closed. The present invention enables the slit illumination to be controlled so as to be constant and uniform throughout the scan, thus is improving the consistency and yield of an IC manufacturing process employing the inventive apparatus.
Preferably said compensation means comprise radiation source control means adapted to control the output of the radiation source (e.g. lamp), which is employed to produce the illumination beam, in accordance with the degree of opening of the beam masking means, to compensate for said variation.
Alternatively or in addition, said compensating means may be such that the beam masking means are constructed of a material substantially transparent to said illumination beam and are shaped so that portions of said illumination beam incident thereon are totally internally reflected.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallisation, oxidation, chemomechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0 067250-4.